Object with radially-varying properties and apparatus and method of preparing the same

ABSTRACT

A method of preparing an object with a radially-varying property is disclosed which includes the steps of providing a preform reactor including an outer container having a bottom, an inner container installed in the outer container, the inner container having a bottom, a rotating rod installed at a position in the outer container, and a sealing member for sealing the outer and inner containers at the bottoms thereof; filling the inner container with an inner material and a space between the inner container and the outer container with an outer material wherein the outer material has a different property from the inner material; removing the inner container; and rotating the rotating rod for laminar mixing of the inner and outer materials. An apparatus for carrying out the inventive method, and objects formed in accordance with the inventive method, are also disclosed.

FIELD OF THE INVENTION

The present invention relates to an object with radially-varyingproperties, which is used to prepare graded-index plastic optical fiberin the field of communication or image transmission. More particularly,the present invention relates to a method of preparing an object withradially-varying properties and an apparatus for preparing the same. Theobject with radially-varying properties can be prepared with polymers orceramics. In this invention, the properties mean an optical propertysuch as refractive index, tensile strength, color, heat expansioncoefficient, relative concentration of components, effect of catalyst,etc.

BACKGROUND OF THE INVENTION

An object with radially-varying properties can be used in the field ofcommunication or image transmission or for other purposes. Inparticular, an object with radially-varying properties has been used asgraded-index plastic optical fiber for telecommunication.

The conventional optical fibers for communication systems are classifiedinto single-mode glass optical fibers and multi-mode glass opticalfibers. The single-mode glass optical fibers have been widely used aslong-distance and high-speed communication media. However, because thesingle-mode glass optical fibers have small core diameters, typically 5to 10 microns, extreme accuracy is required in the alignment of thefibers for interconnection with other components of the opticalcommunication system, thereby increasing the costs of the whole system.In contrast to single-mode glass fibers, multi-mode glass fibers, whichcan have diameters larger than single-mode glass fibers, have been usedprimarily for short distance transmission such as local area networks(LANs). However, even their moderate cost for interconnections haslimited their application. Consequently, metallic cables such as twistedpair or coaxial cable are still used extensively in short rangeapplications, namely up to 200 meters. However, these metallic cablescannot meet the anticipated future bandwidth requirement of severalhundred MHz (for example, the asynchronous transfer mode[ATM] standardof 625 megabits per second). There has been considerable interest indeveloping plastic optical fiber (POF) in the short range communicationapplications, such as LANs. POF can have core diameters of about 0.5 to1.0 mm, which makes it possible to adopt injection-molded polymerconnectors, drastically reducing the cost associated withinterconnecting the POF to the other components of a system. Theseplastic optical fibers can have a step-index (SI) structure orgradient-index (GI) structure. Unfortunately, step-index plastic opticalfiber (SI-POF) suffers high modal dispersion and therefore cannot meetthe bandwidth requirements. However, gradient-index plastic opticalfibers (GRIN-POF), having low modal dispersion, have high potential tobe a high bandwidth, cost effective media for use in short rangecommunication applications.

An interfacial gel polymerization process for preparing GRIN-POF wasintroduced by professor Koike in 1988 (Koike, Y. et al., Applied Optics,vol. 27, 486(1988)), and thereafter many patent applications were filed:U.S. Pat. No. 5,253,323 to Nippon Petrochemicals Co.; U.S. Pat. No.5,382,448 to Nippon Petrochemicals Co.; U.S. Pat. No. 5,593,621 toYasuhiro Koike and Ryo Nihei; International Patent PCT WO 92/03750G02B6/00 to Nippon Petrochemical Co.; International Patent PCT WO92/03751 G02B6/00; Japan Kokai Tokyo Koho JP 03-78706 G02B6/00 toMitsubishi Rayon; Japan Kokai Tokyo Koho JP 04-86603 G02B6/00 to TorayInd., etc. These processes may be divided into two broad types:

1. Batch processes in which a preform is made with a gradient index andsubsequently drawn into a fiber. The preform is made of a polymer(s)plus a low molecular weight additive.

2. Fiber extrusion processes followed by radial extraction of lowmolecular weight components, and/or radial infusion of molecular weightcomponents, and subsequent polymerization of residual monomer.

The first type of process was successfully implemented in producingfiber with the measured bandwidth of 2.5 Gbits/second. The second typeof process has had similar success in achieving a high bandwidth.

In addition to the above-mentioned patents and patent applications, U.S.patent application Ser. No. 89/929,161 (PCT/US97/16172) now U.S. Pat.No. 6,267,915 on a method of preparing GI optical fiber was filed byPark and Walker. The process by Park et al. is achieved by applying apolymeric material having an axial variation of a material property to arotating cone, which converts the axial variation to the radialvariation. Park et al. also disclose an apparatus for producing acylindrical form with at least one radially-varying material propertycomprising mechanical means for transforming an axial variation of amaterial property into a radial properly of the material property.

With respect to using GRIN-POF in LANs and other related applications,the objective is to minimize modal dispersion. The required radialrefractive index profile for minimal modal dispersion has been studiedextensively. The model (Halley,P. [1987] Fiber Optic Systems, J. Wileyand Sons; Olshansky,R., D. B. Keck [976] Appl.Opt.15(2): 483-491) of aGRIN fiber normally considered is that of a “power law” index variation:$\begin{matrix}{{n(r)} = {n_{1}\left\lbrack {1 - {2{\Delta \left( \frac{r}{a} \right)}^{g}}} \right\rbrack}^{\frac{1}{2}}} & {{{for}\quad r} \leq a} \\n_{2} & {{{for}\quad r} > a}\end{matrix}$

where r is the radial distance from the fiber axis, a is the radius ofthe fiber, n₁ and n₂ are the refractive indices at r=0 and r=a,respectively, where n₁ □ n₂. The parameter g determines the indexprofile as a function of radius and 2□=(n₁ ²−n₂ ²)/n₁ ². In theparticular case where g=2, the power law is called the “parabolic law”.When the value of g approaches to 2, an optimum refractive index profilefor maximum bandwidth can be obtained. It can be shown that if a lightsignal in the form of a delta function is launched into a GRIN fiber,the maximum bandwidth, B is given by:$B = {{\frac{c}{0.088\quad {Ln}_{1}} \cdot \frac{1}{\Delta^{2}}}\quad \left( {{bits}\text{/}{second}} \right)}$

where L is the length of the fiber, and c is the speed of light.

In theory, the bandwidth of GRIN-POF is extremely sensitive to the valueof g near the optimum value. Therefore, in preparing GRIN-POF, how largea bandwidth GRIN-POF has depends on the ability of a process to controlthe value of g. In conventional processes of preparing GRIN-POF, exceptthe process by Park et al., the refractive index profile in the radialdirection is determined by the diffusion of a lower molecular materialor the relative reactivity of two materials. Thus the conventionalprocesses do not have the ability to control the value of g or theradial profile of the refractive index. The process by Park et al.above-mentioned claims to have the ability to control the value of g bymechanical mixing of two or more polymers using a particular extrusionmold die. However, the process has disadvantages in that it is difficultto produce optical fiber with a low attenuation due to the complicatedstructure of the extrusion die and contaminants resulting from thethermal decomposition of polymers from coextrusion process.

Therefore, the present inventors have developed a method of preparing aplastic optical fiber and an apparatus for preparing the same. Theprocess of this invention has the ability to control the refractiveindex profile. Unlike the process of Park et al., the new process is notan extrusion process and the apparatus is not complicated. The presentprocess can provide a method of preparing a plastic optical fiber with alow intensity loss of a light signal as in the process by Koike.

A feature of the present invention is the provision of an object withradially-varying properties, which is used to prepare graded-indexplastic optical fiber in the field of communication or imagetransmission.

Another feature of the present invention is the provision of a method ofpreparing an object with radially-varying properties, which is used toprepare graded-index plastic optical fiber in the field of communicationor image transmission.

A further feature of the present invention is the provision of a methodof preparing an object with radially-varying properties, which has theability to control the value of g or radial index of refraction.

A further feature of the present invention is the provision of anapparatus for preparing an object with radially-varying properties,which has a simple structure.

A further feature of the present invention is the provision of a methodof preparing a plastic optical fiber with a low intensity loss of alight signal.

Other features and advantages of this invention will be apparent fromthe ensuing disclosure and appended claims.

SUMMARY OF THE INVENTION

The method of preparing an object with radially-varying properties inaccordance with the present invention, which is used to preparegraded-index plastic optical fiber in the field of communication orimage transmission, comprises providing a reaction apparatus comprisingan outer container, an inner container installed in the outer container,a rotating rod installed at a position in the inner container, and asealing member for sealing the outer and inner containers at the bottomsthereof; filling the inner container with an inner material and a spacebetween the inner container and the outer container with an outermaterial wherein the outer material has different properties from theinner material; removing the inner container; and rotating the rotatingrod for laminar mixing of the two materials. The apparatus for preparingan object with radially-varying properties in accordance with thepresent invention comprises an outer container with a certaincross-section; an inner container with a certain cross-section,installed in the outer container; a rotating rod installed at a positionin the inner container, and a sealing member for sealing the outer andinner containers at the bottoms thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a profile of a radially varying property and FIG. 1b is aschematic view of an object with radially-varying properties;

FIG. 2 is a schematic view of an apparatus for preparing an object withradially-varying properties according to the present invention;

FIG. 3 is a cross-sectional view of the apparatus of FIG. 2;

FIG. 4 is a schematic view of the sealing member of the apparatus ofFIG. 2 at the bottom;

FIG. 5 is a cross-sectional view of an apparatus according to thepresent invention, showing the mixing ratio of inner material to outermaterial in which the cross-section of the inner container is a heartshape;

FIG. 6a is a cross-sectional view of an apparatus according to thepresent invention, showing the mixing ratio of inner material to outermaterial in which the cross-section of the inner container is a pie-likeshape;

FIG. 6b is a cross-sectional view of an apparatus according to thepresent invention, showing the mixing ratio of inner material to outermaterial in which the cross-section of the inner container is atrumpet-like shape;

FIG. 7a is a cross-sectional view of an the apparatus according to thepresent invention where the cross-sections of the inner and outercontainers are circular type and the circles are concentric;

FIG. 7b is a cross-sectional view of an apparatus according to thepresent invention where the cross-sections of the inner and outercontainers are circular type and the circles are eccentric;

FIG. 8a is a schematic view of an outer container having a cross-sectionof a rectangular type;

FIG. 8b is a schematic view of an outer container having a cross-sectionof a triangular type;

FIG. 9a is a cross-sectional view of an object with radially-varyingproperties according to the present invention, showing the contours withthe same properties when the outer container is a rectangular type; and

FIG. 9b is a cross-sectional view of an object with radially-varyingproperties according to the present invention, showing the contours withthe same properties when the outer container is a triangular type.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The object with radially-varying properties in accordance with thepresent invention, which is used to prepare graded-index plastic opticalfiber in the field of communication or image transmission, can beprepared by a method comprising providing a reaction apparatuscomprising an outer container, an inner container installed in the outercontainer, a rotating rod installed at a position in the innercontainer, and a sealing member for sealing the outer and innercontainers at the bottoms thereof; filling the inner container with aninner material and the space between between the inner container and theouter container with an outer material wherein the outer material hasdifferent properties from the inner material; removing the innercontainer; and rotating the rotating rod for laminar mixing of the twomaterials.

The apparatus for preparing an object with radially-varying propertiesin accordance with the present invention comprises an outer container 1with a certain cross-section; an inner container 2 with a certaincross-section, installed in the outer container; a rotating rod 3installed at a position in the inner container, and a sealing member 4for sealing the outer and inner containers at the bottoms thereof. Ingeneral, the outer container has a cross-section of a circular type,however, the cross section can be a triangular type, a rectangular type,or other geometrical shape depending on the profile of properties of anobject. The present invention shall be described in detail withreference to the accompanying drawings.

FIG. 1a is a profile of a radially varying property and FIG. 1b is aschematic view of an object with radially-varying properties. FIG. 2 isa schematic view of an apparatus for preparing an object withradially-varying properties according to the present invention. FIG. 3is a cross-sectional view of the apparatus of FIG. 2. FIG. 4 is aschematic view of the sealing member of the apparatus of FIG. 2 forsealing the outer and inner containers at the bottoms.

FIG. 5 is a cross-sectional view of an apparatus according to thepresent invention, showing the mixing ratio of inner material to outermaterial in which the cross-section of the inner container is a heartshape. FIG. 6a is a cross-sectional view of the apparatus according toan present invention, showing the mixing ratio of inner material toouter material in which the cross-section of the inner container is apie-like shape, and FIG. 6b is a cross-sectional view of an apparatusaccording to the present invention, showing the mixing ratio of innermaterial to outer material in which the cross-section of the innercontainer is a trumpet-like shape. FIG. 7a is a cross-sectional view ofan apparatus according to the present invention where the cross-sectionsof the inner and outer containers are circular type and the circles areconcentric, and FIG. 7b is a cross-sectional view of an apparatusaccording to the present invention where the cross-sections of the innerand outer containers are circular type and the circles are eccentric.

FIG. 8a is a schematic view of an outer container having a cross-sectionof a rectangular type, and FIG. 8b is a schematic view of an outercontainer having a cross-section of a triangular type. FIG. 9a is across-sectional view of an object with radially-varying propertiesaccording to the present invention, showing the contours with the sameproperties when the outer container is a rectangular type, and FIG. 9bis a cross-sectional view of an object with radially-varying propertiesaccording to the present invention, showing the contours with the sameproperties when the outer container is a triangular type.

As shown in FIGS. 2 and 4, the outer container 1, inner container 2 androtating rod 3 are assembled into the grooves 7, 8 and 9 on the sealingmember 4, respectively. The outer container 1, inner container 2 androtating rod 3 should be vertically parallel. The rotating rod can beassembled at a position between the inner container and the outercontainer. The outer container and inner container should be sealed withthe sealing member. An inner material is filled in the inner containerand an outer material is filled in the space between the inner containerand the outer container, wherein the inner material has a high index ofrefraction and the outer material has a low index of refraction. Theapparatus can include a stirring means to agitate the inner material anda stirring means to agitate the outer material. Further, the apparatuscan include a heating means to heat the inner and outer materials. Theapparatus can have a second inner container or more in the outercontainer. The outer container has a diameter of about 15 cm and therotating rod has a diameter of from 5 mm to 5 cm and an rpm of from 5 to30. The outer container, inner container-and rotating rod are made of amaterial selected from the group consisting of stainless steel,hastelloy, brass, aluminum, TEFLON, glass and ceramic. The innercontainer, outer container and rotating rod have a cross-sectionselected from the group consisting of triangular, rectangular,pentagonal, hexagonal, circular, oval, heart shape, trumpet shape andother geometrical shapes. The rotating rod can be assembled at aposition between the inner container and the outer container.

The inner and outer materials can be liquid, thermoplastic polymerswhich have been completely polymerized and heated up to or above themelting points or the glass transition temperatures, liquid prepolymersor oligomers which have not completed polymerization reaction, monomers,or suspensions of ceramic particles. It is preferable that thedifferences in the density and viscosity are small between the twomaterials, also it is preferable that the viscosities are not very low.In case that the viscosity of the material is low, it is preferable tocarry out polymerization to increase the viscosity up to a desired levelby heating the apparatus or radiating ultraviolet thereon.

When the inner material in the inner space 5 and the outer material inthe outer space 6 (i.e., the space between the inner container 2 and theouter container 1) come to a standstill, the inner container is slowlyremoved upward not to disturb the inner and outer materials. If theinner container is removed, the solid wall between the inner and outermaterials does not exist any more. Consequently, the two materials canbe mixed due to gravity thereof. However, if the difference in thedensity between the two materials is small and if the viscosities of thetwo materials are not very low, the interface between the two materialstend to remain unperturbed by gravity. After the inner container isremoved, the rotating rod 3 is rotated by a motor (not shown). A motoris connected to the rotating rod, which can be easily carried out by oneskilled in the art. When the rotating rod is slowly rotated, the innerand the outer materials are mixed in the circumferential direction ofthe rotating rod, but not disturbed in the radial direction or in theaxial direction, because the viscous force is dominant. This mixing flowof the inner and the outer materials in the circumferential direction isknown as “laminar flow” in fluid mechanics. Thus the mixing of the innerand the outer materials in the circumferential direction in the presentinvention may be called the “laminar mixing”.

As the rotating rod rotates, the inner and the outer materials are mixedmore and more uniformly in the circumferential direction, varying themixing ratio of the inner material to the outer material so as toprepare an object with radially-varying properties from the center tothe outer circumference. FIG. 5 is a cross-sectional view of theapparatus according to the present invention, showing the mixing ratioof inner material to outer material. At the radial position of R₁ (i.e.,r=R₁) from the center, the mixing ratio of the inner material to theouter material is the ratio of lengths of the arcs, thus being L₁ to L₂.As shown in FIG. 5, at a smaller value of the radial position the lengthof the arc corresponding to the amount of the inner material is larger,thus the concentration of the inner material is higher than that of theouter material at that radial position. On the other hand, at a largervalue of the radial position-the corresponding arc length is smaller.Thus the concentration of the inner material is relatively lower at thatradial position.

After completing laminar mixing of the inner and the outer materials,the rotating rod is slowly removed upward from the inner containerwithout disturbing the mixed material. The means for removing therotating rod upwardly from the inner container can be easily understoodby one skilled in the art. The rotating rod can be removed while it iskept rotating or stopped rotating. Once the rotating rod is removed fromthe apparatus, an object is prepared, which has radially-varyingproperties from the center to the outer surface, but has a uniformproperty at each radial position. The space occupied by the rotating rodis filled with the mixed material as it is removed. The total volume ofthe rotating rod is relatively small compared to the volume of the innermaterial or the outer material, therefore the error in the mixing ratioat the center is negligible.

Alternatively, in the present invention, the entire apparatus can berotated during or after the laminar mixing. The apparatus may be calleda “preform reactor”. In the case that the density difference between theinner and the outer materials is significant, the laminar mixed layercan be disturbed due to the gravity effect. Such disturbance can beprevented by rotating the preform reactor. If the preform reactor isrotated, the centrifugal force imposed on the mixed material offsets thegravity force. The rotational speed of the preform reactor can bedetermined depending on the density difference between the inner and theouter materials, which can be easily understood by one skilled in theart. Preferably, the rotational speed is in the range of 50 to 2,000rpm, more preferably 50 to 500 rpm.

The laminar-mixed material thus prepared in the outer container is in aliquid state, and should be changed into a solid state. If the mixedmaterial is a polymer which has been heated up to or above the meltingpoint or glass transition temperature, the material can be cooled slowlyto a solid state. If the material is a prepolymer, polymerization shouldproceed by heating or radiating ultraviolet and the material should bethen cooled slowly. If the material is a green form of ceramicsuspensions, a solid state object can be obtained by firing in afurnace.

The property of an object prepared in accordance with the presentinvention can be an optical property, index of refraction, heatexpansion coefficient, porosity, etc.

Control of Mixing Ratio in Radial Direction

In FIG. 5, the arc length L₁ corresponds to the amount of the innermaterial at the radial position, whereas L₂ corresponds to the amount ofthe outer material at the same radial position. R₁, and θ₁ is the anglecorresponding to the arc length L₁. Thus the ratio of the inner materialto the outer material at R₁ is equivalent to the ratio of L₁ to L₂ or θ₁to 360−θ₁. At a radial position smaller than R₁, the arc lengthcorresponding to the amount of the inner material is larger than L₁ orthe angle is larger than θ₁. Thus the relative concentration of theinner material to the outer material is larger. At a radial positionlarger than R₁, on the other hand, the arc length is larger than L₁ orthe angle is larger than θ₁, so that the relative concentration ratio ofthe inner material can be high. Therefore, in the case of FIG. 5 inwhich the cross-section of the inner container is a heart-like shape, acylindrical object that has gradually decreasing concentration of theinner material in the radial direction can be prepared. If the index ofrefraction of the inner material is higher than that of the outermaterial, the cylindrical object can have the refractive index profilesimilar to the one shown in FIG. 1.

As shown in FIG. 6(a), if the cross-section of the inner container is apie-like shape, the angle θ₁ corresponding to the amount of the innermaterial is constant regardless of the radial position. Thus the mixingratio of the two materials is the same over all radial positions. On theother hand, if the cross-section of the inner container is atrumpet-like shape as shown in FIG. 6(a), the angle θ₁ corresponding tothe amount of the inner material becomes increasingly larger as theradial position increases. Consequently, a cylindrical object that has agradually increasing concentration of the inner material in the radialdirection can be prepared.

If the cross-section of the inner container is a concentric circle withthat of the outer container as shown in FIG. 7(a), the two materials arenot mixed at all. In this case, the cylindrical object will become acylindrical core with radius of a surrounded uniformly by the outermaterial of a thickness of t. On the other hand, if the inner circle isnot positioned concentrically with the outer circle as shown in FIG.7(b), the cylindrical object will consist of three layers where a pureinner material with a radius of a is surrounded by a mixture at a radialposition from a to b, which is then surrounded by a pure outer materialbeyond the radial position b.

Thus, the mixing ratio of the inner material to the outer material canbe controlled by varying the cross-section and/or the location of theinner container, which is one of the most important features of thepresent invention.

Cross-sectional Shape of the Outer Container

The cross-section of the outer container will be a circular shape inmost applications. However, a rectangular or triangular shape can beapplied whenever necessary as shown in FIGS. 8(a) and 8(b), or othervarious cross-sectional shapes can be also applied. In case that thecross-section of the outer container is a rectangular or triangularshape as shown in FIGS. 8(a) and 8(b), the physical properties of theobjects are varied as shown schematically in FIGS. 9(a) and 9(b). InFIGS. 9(a) and 9(b), the contour lines are the loci of the equal mixingratio of the two materials. While the contour lines are of circularshape near the center, they become closer to the shape of the outercontainer as the outer container wall is approached.

Preparation of Objects with Radially-Varying Properties

According to the present invention, an object with radially-varyingproperties and subsequently a graded-index plastic optical fiber can beprepared as follows. A monomer mixture of 65 weight % ofmethylmethacrylate (MMA) and 35 weight % benzylmethacrylate (BMA) is putinto the inner container as shown in FIG. 2, and a monomer mixture of 80weight % of methylmethacrylate (MMA) and 20 weight % benzylmethacrylate(BMA) is put into the outer container. The overall apparatus is heatedup to 70° C. and kept at that temperature to copolymerize the monomers.Although not shown in FIG. 2, a stirrer or an agitator can be installedin the apparatus to proceed with polymerization of the monomers.

After polymerization proceeds to a point where the viscosity of theprepolymer is about 1000 to 2000 centipoises, the inner container isslowly removed upward. The rotating rod is then rotated so as to mix thetwo prepolymers in the circumferential direction. While the rotationalspeed of the rotating rod can be any value as long as the flow is notinfluenced by the inertia in radial direction, it preferably is in therange of about 5 to 30 rpm.

After completion of mixing the two materials by rotating the rod, therod is lowly removed upward. At this stage, the material in the outercontainer is in a state where the mixing in the circumferentialdirection is uniform at every radial position, and the mixing ratiovaries in the radial direction. In this case, the mixed material has arefractive index profile equivalent to 35 weight % concentration of BMAnear the center and 20 weight % concentration of BMA near the wall ofthe outer container. Polymerization of the prepolymers is continued at75° C. and is completed at 125° C. Unreacted residual monomers may bemixed by the molecular diffusion due to the difference in theconcentration of the two prepolymers. However, the diffusion of theunreacted monomers can be neglected, because the viscosities of theprepolymers are relatively high and the difference of concentrations ofthe prepolymers is relatively small.

After completion of the polymerization, the MMA-BMA copolymer is cooledto or below the glass transition temperature and the outer container isremoved to obtain a solid cylindrical object. As MMA and BMA havesimilar reactivity, the object thus obtained is an amorphous randomcopolymer, and the concentration of BMA varies from about 35% to 20%along the radial direction. The index of refraction of the object is1.519 at the center where the concentration ratio of MMA/BMA is 65%/35%,and decreases to 1.507 at the outer surface of the object where theconcentration ratio is 80%/20%.

In the case of FIG. 5 in which the cross-section of the inner containeris a heart-like shape, the cylindrical object shows a refractive indexprofile of a parabolic shape as shown in FIG. 1.

The cylindrical objects prepared in accordance with the presentinvention can be transformed to a GI plastic optical fiber (GI-POF) bythermal drawing, or to a relatively thick strand to prepare a GI rodlens.

If the cross-section of the inner container is a trumpet-like shape asshown in FIG. 6(a) or if the inner material is exchanged with the outermaterial, the angle θ₁ corresponding to the amount of the inner materialbecomes increasingly larger as the radius R₁ increases, thus acylindrical object that has gradually increasing concentration of theinner material in the radial direction can be prepared. The index ofrefraction of such object increases from the center to the outersurface. These objects can be used to prepare a negative gradientoptical lens for the correction of aberration.

Apparatus for Preparing Objects

The apparatus shown in FIG. 2 can be prepared from a small size to alarge scale regardless of the diameter or the length of the outercontainer. However, if a polymerization reaction is to be induced, themaximum diameter of the outer container is preferably smaller than 15 cmto facilitate heat transfer. The maximum length of the outer containeris preferably smaller than 150 cm if the conventional thermal drawing isto be applied afterward.

The rotating rod can have a diameter of about 5 mm to 5 cm, preferablyabout 5 mm to 1 cm. The rotating rod 3 is installed in the innercontainer 2 or between the inner container and the outer container 1.The profile of physical properties of an object can be also varied bythe location of the rod. The rod can be rotated by an electric orphysical means, which will be easily carried out by one skilled in theart.

A stirring means may be further added in the inner container or outercontainer to stir the contents, and a heating means may be alsoinstalled in the inner container or outer container to heat thecontents.

Two or more inner containers can be installed in the outer containerwhose cross section may be identical or different.

The outer container, inner container and rotating rod can be made ofstainless steel, hastelloy, brass, aluminum, TEFLON, glass or ceramic,which is easily understood by one skilled in the art.

Inner and Outer Materials

Monomers, homopolymers, copolymers or mixtures thereof can be used asthe inner and the outer materials.

Examples of the monomers are methylmethacrylate, benzylmethacrylate,phenylmethacrylate, 1-methylcyclohexylmethacrylate,cyclohexylmethacrylate, chlorobenzylmethacrylate,1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate,diphenylethylmethacrylate, furfurylmethacrylate,1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate,pentabromophenylmethacrylate, styrene,perfluoro-2,2-dimethyl-1,3-dioxole, tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexafluoropropylene,trifluoroethylene, perfluoroallyl vinyl ether, and fluorovinyl ether.

Examples of the homopolymers are polymers polymerized from the monomersabove.

Examples of the copolymers are copolymer ofmethylmethacrylate(MMA)-benzylmethacrylate(BMA), copolymer ofstyrene-acrylonitrile(SAN), copolymer ofmethylmethacrylate(MMA)-2,2,2-trifluoroethylmethacrylate(TFEMA),copolymer ofmethylmethacrylate(MMA)-2,2,3,3,3-pentafluoropropylmethacrylate(PFPMA),copolymer ofmethylmethacrylate(MMA)-1,1,1,3,3,3,-hexafluoroisomethacrylate(HFIPMA),copolymer ofmethylmethacrylate(MMA)-2,2,3,3,4,4,4-heptafluorobutylmethacrylate(HFBMA),copolymer oftrifluoroethylmethacrylate(TFEMA)-pentafluoropropylmethacrylate(PFPMA),copolymer oftrifluoroethylmethacrylate(TFEMA)-hexafluoroisomethacrylate(HFIPMA), andcopolymer oftrifluoroethylmethacrylate(TFEMA)-heptafluorobutylmethacrylate(HFBMA).

Further examples of the copolymers include copolymers polymerized with afirst monomer of perfluoro-2,2-dimethyl-1,3-dioxole and a second monomerselected from the group consisting of tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexa-fluoropropylene,trifluoroethylene, perfluoroallyl vinyl ether, and fluorovinyl ether.

Further examples of the copolymers include terpolymers polymerized witha first monomer of perfluoro-2,2-dimethyl-1,3-dioxole and a secondmonomer and a third monomer which are selected from the group consistingof tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride,hexa-fluoropropylene, trifluoroethylene, perfluoroallyl vinyl ether, andfluorovinyl ether.

Further examples of the copolymers include copolymers polymerized with afirst monomer of perfluoroallyl vinyl ether and a second monomerselected from the group consisting ofperfluoro-2,2-dimethyl-1,3-dioxole, tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexa-fluoropropylene,trifluoroethylene, and fluorovinyl ether.

Further examples of the copolymers include terpolymers polymerized witha first monomer of perfluoroallyl vinyl ether and a second monomer and athird monomer which are selected from the group consisting ofperfluoro-2,2-dimethyl-1,3-dioxole, tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexa fluoropropylene,trifluoroethylene, and fluorovinyl ether.

A material with a higher index of refraction is put into the innercontainer as the inner material, and a material with a lower index ofrefraction is put into the space between the inner container and theouter container as the outer material.

The radially-varying properties of an object include allphysico-chemical properties such as tensile strength, color, thermalexpansion coefficient, catalytic activity, and porosity as well as theindex of refraction of POF. An object in which these properties varyradially can be prepared by the present invention once proper materialsare chosen.

In addition, a ceramic material can be applied in this invention. Forexample, if the concentration of ceramic particles is increased alongthe outward direction in a matrix, a state-of-the-art product with anexcellent-heat resistance and abrasion resistance can be-obtained. Asanother example, if two ceramic suspensions are used as inner and outermaterials, a product obtained with good hardness at the surface withminimal influence by the thermal stress resulting from the differencesin the thermal expansion coefficient. The examples of the ceramicsuspensions are alumina and zirconium. Such ceramic materials withradially-varying properties are known as Functionally Gradient Materials(FGMs).

EXAMPLE Preparation of Objects with Radially-Varying Properties

According to the present invention, an object with radially-varyingproperties and subsequently a graded-index plastic optical fiber can beprepared as follow: A monomer mixture consisting of 64.65 weight % ofmethylmethacrylate (MMA), 34.65 weight % of benzylmethacrylate (BMA),0.5 weight % of benzoyl peroxide (BPO) as initiator of polymerization,and 0.2 wight % of n-butane thiol as chain transfer agent were put intothe inner container. A monomer mixture consisting of 79.65 weight % ofmethylmethacrylate (MMA), 19.65 weight % of benzylmethacrylate (BMA),0.5 weight % of benzoyl peroxide (BPO) as initiator of polymerization,and 0.2 wight % of n-butane thiol as chain transfer agent were put intothe outer container. The entire apparatus was heated up to 70° C. andkept at that temperature to copolymerize the monomer mixtures.

After polymerization proceeded to a point where the viscosity of theprepolymer is about 1500 centipoises, the inner container was slowlyremoved upward. The rotating rod was then rotated so as to mix the twoprepolymers in the circumferential direction. The rotational speed ofthe rotating rod was 20 rpm.

After completion of mixing the two materials by rotating the rod, therod was slowly removed upward. Then the material in the outer containerwas in a state where the mixing in the circumferential direction isuniform, and the mixing ratio varies in the radial direction. In thiscase, the mixed material has a profile equivalent to 35 weight %concentration of BMA near the center and 20 weight % concentration ofBMA near the wall of the outer container. Polymerization of theprepolymers was continued at 75° C. and was completed at 125° C.

After completion of the polymerization, the MMA-BMA copolymer was cooledbelow the glass transition temperature and the outer container wasremoved to obtain a solid cylindrical object. As MMA and BMA havesimilar reactivity, the object thus obtained was an amorphous randomcopolymer, and the concentration of BMA varied from about 35% to 20%along the radial direction. The index of refraction of MMA-BMA copolymerwas 1.519 at the center where the concentration ratio of MMA BMA was65%/35%, and decreased to 1.507 at the outer surface of the object wherethe concentration ratio of MMA/BMA was 80%/20%.

The cylindrical objects prepared in accordance with the presentinvention were transformed to a GI plastic optical fiber (GI-POF) bythermal drawing, and to a relatively thick strand to prepare a GI rodlens.

It should be apparent to those ordinarily skilled in the art thatvarious changes and modifications can be added without departing fromthe scope of the present invention.

What is claimed is:
 1. A method of preparing an object with aradially-varying property, which comprises the steps of: providing apreform reactor comprising an outer container having a bottom, an innercontainer installed in said outer container, said inner container havinga bottom, a rotating rod installed at a position in said outercontainer, and a sealing member for sealing said outer and innercontainers at said bottoms thereof; filling said inner container with aninner material and a space between said inner container and said outercontainer with an outer material wherein said outer material has adifferent property from said inner material; removing said innercontainer; and rotating said rotating rod for laminar mixing of saidinner and outer materials.
 2. The method according to claim 1 furthercomprising the step of rotating said preform reactor during or aftersaid laminar mixing by said rotating rod.
 3. The method according toclaim 2 wherein said preform reactor rotates with a rotational speed inthe range of 50 to 2,000 rpm.
 4. The method according to claim 1 whereinsaid inner container, said outer container and said rotating rod have across-section selected from the group consisting of triangular,rectangular, pentagonal, hexagonal, circular, oval, heart shape and,trumpet shape.
 5. The method according to claim 1 wherein said rotatingbody is installed at a position between said inner container and saidouter container.
 6. The method according to claim 1 wherein said innermaterial and said outer material are selected from the group consistinga monomer, a homopolymer, a copolymer, and a mixture thereof.
 7. Themethod according to claim 6 wherein said monomer is selected from thegroup consisting of methylmethacrylate, benzylmethacrylate,phenylmethacrylate, 1-methylcyclohexylmethacrylate,cyclohexylmethacrylate, chlorobenzylmethacrylate,1-phenylethylmethacrylate, 1,2-diphenylethylmethacrylate,diphenylethylmethacrylate, furfurylmethacrylate,1-phenylcyclohexylmethacrylate, pentachlorophenylmethacrylate,pentabromophenylmethacrylate, styrene,perfluoro-2,2-dimethyl-1,3-dioxole, tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexa-fluoropropylene,trifluoroethylene, perfluoroallyl vinyl ether, and fluorovinyl ether. 8.The method according to claim 6 wherein said homopolymer is a polymerwhich is polymerized from a monomer selected from the group consistingof methylmethacrylate, benzylmethacrylate, phenylmethacrylate,1-methylcyclohexylmethacrylate, cyclohexylmethacrylate,chlorobenzylmethacrylate, 1-phenylethylmethacrylate,1,2-diphenylethylmethacrylate, diphenylethylmethacrylate,furfurylmethacrylate, 1-phenylcyclohexylmethacrylate,pentachlorophenylmethacrylate, pentabromophenylmethacrylate, styrene,perfluoro-2,2-dimethyl-1,3-dioxole, tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexa-fluoropropylene,trifluoroethylene, perfluoroallyl vinyl ether, and fluorovinyl ether. 9.The method according to claim 6 wherein said copolymer is selected fromthe group consisting of copolymer ofmethylmethacrylate(MMA)-benzylmethacrylate(BMA)], copolymer ofstylene-acrylonitrile(SAN), copolymer ofmethylmethacrylate(MMA)-2,2,2-trifluoroethylmethacrylate(TFEMA),copolymer ofmethylmethacrylate(MMA)-2,2,3,3,3-pentafluoropropylmethacrylate(PFPMA),copolymer ofmethylmethacrylate(MMA)-1,1,1,3,3,3,-hexafluoroisomethacrylate(HFIPMA),copolymer ofmethylmethacrylate(MMA)-2,2,3,3,4,4,4-heptafluorobutylmethacrylate(HFBMA),copolymer oftrifluoroethylmethacrylate(TFEMA)-pentafluoropropylmethacrylate(PFPMA),copolymer oftrifluoroethylmethacrylate(TFEMA)-hexafluoroisomethacrylate(HFIPMA), andcopolymer oftrifluoroethylmethacrylate(TFEMA)-heptafluorobutylmethacrylate(HFBMA).10. The method according to claim 6 wherein said copolymer is selectedfrom the group consisting of copolymers polymerized with a first monomerof perfluoro-2,2-dimethyl-1,3-dioxole and a second monomer selected fromthe group consisting of tetrafluoroethylene, chlorotrifluoroethylene,vinylidene fluoride, hexa-fluoropropylene, trifluoroethylene,perfluoroallyl vinyl ether, and fluorovinyl ether.
 11. The methodaccording to claim 6 wherein said copolymer is selected from the groupconsisting of terpolymers polymerized with a first monomer ofperfluoro-2,2-dimethyl-1,3-dioxole and a second monomer and a thirdmonomer which are selected from the group consisting oftetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride,hexafluoropropylene, trifluoroethylene, perfluoroallyl vinyl ether, andfluorovinyl ether.
 12. The method according to claim 6 wherein saidcopolymer is selected from the group consisting of copolymerspolymerized with a first monomer of perfluoroallyl vinyl ether and asecond monomer selected from the group consisting ofperfluoro-2,2-dimethyl-1,3-dioxole, tetrafluoroethylene,chlorotrifluoroethylene, vinylidene fluoride, hexafluoropropylene,trifluoroethylene, and fluorovinyl ether.
 13. The method according toclaim 6 wherein said copolymer is selected from the group consisting ofterpolymers polymerized with a first monomer of perfluoroallyl vinylether and a second monomer and a third monomer which are selected fromthe group consisting perfluoro-2,2-dimethyl-1,3-dioxole,tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride,hexafluoropropylene, trifluoroethylene, and fluorovinyl ether.
 14. Themethod according to claim 6 wherein said inner material and outermaterial are a liquid metal or a ceramic suspension.
 15. The methodaccording to claim 1 wherein said property is an optical property. 16.The method according to claim 1 wherein said property is the index ofrefraction.
 17. The method according to claim 1 wherein said property isthe thermal expansion coefficient.
 18. The method according to claim 1wherein said property is porosity.
 19. The method according to claim 1wherein said property is the index of refraction and wherein said innermaterial has a higher index of refraction than said outer material. 20.The method according to claim 1 further comprising removing saidrotating rod after said laminar mixing.
 21. The method according toclaim 1 wherein said rotating rod is installed in said inner container.